The present invention relates generally to a process for the production of a superabsorber, a device for the production of a superabsorber, superabsorbers, a composite, a process for the production of a composite, the composites obtainable by this process, chemical products, such as foams, shaped articles or fibers, and the use of a superabsorber.
Superabsorbers are water-insoluble, crosslinked polymers which are capable of absorbing, and retaining under a certain pressure, large amounts of aqueous liquids, in particular body fluids, preferably urine or blood, with swelling and formation of hydrogels. Due to these characteristic properties these polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby diapers, incontinence products or sanitary napkins.
Superabsorbers are as a rule produced by free-radical polymerization of monomers carrying acid groups in the presence of crosslinking agents. In this context polymers having different absorber properties can be prepared by the choice of the monomer composition, the crosslinking agent and the polymerization conditions and the processing conditions for the hydrogel obtained after the polymerization. Further possibilities are offered by the preparation of graft polymers, for example using chemically modified starch, cellulose and polyvinyl alcohol in accordance with DE-OS 26 12 846.
In order to optimize the properties, in particular the absorption properties, of superabsorbers for use in hygiene articles, the polymer particles obtained after comminution of the dried polymer gel are modified, preferably modified on the surface. During the surface modification a core-shell-like morphology may form, which is preferred in particular for superabsorber particles. This modification serves, for example, to provide the superabsorbers with odor-binding properties, to improve the dust properties of the superabsorbers, to reduce caking together of the superabsorber particles, to improve the absorption capacity of the superabsorbers under a pressure load and/or to influence advantageously the permeability properties of the superabsorber.
During these modification measures the superabsorbent polymer particles are brought into contact with a treatment agent, which can be present in a solution or dispersion or as a powder. In order to achieve as uniform as possible a treatment of the surface of the superabsorbers with the treatment agent, this bringing into contact is conventionally carried out by a procedure in which the superabsorber particles are initially introduced into a mixer and the treatment agent is then added with intensive mixing of the polymer particles. In the case of surface post-crosslinking as the surface modification measure, conventional mixing devices, such as, for example, the Patterson-Kelley mixer, the Schugi® vertical mixer (e.g. from Hosokawa with the trade name “Flexomix”), the DRAIS® turbulence mixer, the Lödige® mixer, the Ruberg® mixer, the screw mixer, the plate mixer or the fluidized bed mixer, are employed for this. Further suitable mixing devices are described, for example, in chapter 3.2.8.1 of the book “Modern Superabsorbent Polymer Technology” by F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998.
In mixing devices which are conventionally employed, if aqueous solutions are employed as treatment agents the superabsorbers treated with these tend to form lumps immediately after leaving the mixing device. An inhomogeneous distribution of the treatment agent in or on the superabsorber is also often observed. In order to prevent such formation of lumps and to improve the homogenization, after leaving the mixing device the superabsorber is stored, for example, in conical silos and agitated by means of a rotating screw (cone mixer). The superabsorbers are stored in such a cone mixer until they are sufficiently relaxed and homogenized, and in particular have been converted from a ground-damp state which tends to stick together into a free-flowing state. Sufficient relaxation or homogenization is achieved, for example, when the superabsorber has reached a free-flowing state again. This is often the case when about the same flowability value which the superabsorbers showed before the addition of the aqueous treatment agent is reached. At this point in time the added liquid is distributed essentially homogeneously within the superabsorber particles, so that the difference between the water content in the core and the water content in the surface region (if charging is carried out with an aqueous treatment agent) of the polymer particles is increasingly compensated.
Disadvantages of such processes are, however, that a sufficient relaxation and homogenization can be achieved only very slowly, and in particular relatively large amounts of treated superabsorber cannot be processed. Due to these limitations the maximum amount of liquid which can be added to the superabsorber during the modification is also limited.
Furthermore, significant mechanical damage to the superabsorber may easily occur due to the rotating screw in the cone mixer or in other mixers of the rotor-stator principle, as a result of which the absorption properties are adversely influenced and in particular the dust content is increased.